This invention relates to pressure vessels having at least one removable end closure for providing full bore access to install and remove cylindrical semipermeable membrane cartridges. More particularly, the invention relates to providing pressure vessels for receiving and operating a plurality of cylindrical semipermeable membrane cartridges in a column to treat a liquid aqueous feed supplied thereto at superatmospheric pressure as part of a cross-flow filtration system.
Cylindrical pressure vessels for holding one or multiple cylindrical cartridges useful in cross-flow filtration are shown in a number of U.S. patents including U.S. Pat. Nos. 4,717,035, 4,517,085, 4,781,830, 5,866,001 and 5,720,411. Cross-flow filtration is a type of separation using semipermeable membranes where only a portion of the feed liquid passes through the semipermeable membrane, with the remainder of the liquid flowing across the membrane and exiting from the other end of the filtration cartridge. In such an arrangement, there are two exit ports from the pressure vessel, i.e. one for the remainder of the now concentrated feed stream and one for the permeate.
When multiple cylindrical cartridges are included within a single pressure vessel, the liquid feed will generally enter one end and then flow serially through all of the cartridges exiting at the opposite end of the pressure vessel while the permeate flows spirally inward to a central porous permeate tube. In such an arrangement, each cartridge will have an open anti-telescoping plate (ATP) at each end, and some type of a connector will interconnect the permeate tubes of adjacent cartridges to create a combined permeate flow path centrally of the pressure vessel. Exemplary connectors 22 are shown in the ""085 patent. In order to save space between cartridges in a column within a pressure vessel, such couplings are being designed to reside substantially entirely within the permeate tube, thus minimizing the distance between ATPs of adjacent cartridges.
All such connections in a pressure vessel subjected to superatmospheric pressure must of course contain suitable seals to prevent leakage. Generally O-ring seals, as shown at 49 in the ""830 patent are used, but elastomeric seals of square cross-section, such as the item 117 in the ""411 patent, have also been used. Effective seals have always been very important in cross-flow filtration operations to prevent the feed liquid from leaking into and contaminating the permeate. They have, however, become of even more importance when cross-flow filtration is carried out using multiple cartridges in one pressure vessel, as for the purification of seawater or the like, where leakage of such seals requires a shut-down of a pressure vessel and removal of cartridges and replacement of leaking seals which is expensive both in manpower and in loss of operational time. Because 40- or 60-inch long cartridges must be allowed some tolerance in longitudinal length and because such tolerances can be cumulative with respect to pressure vessels for holding multiple cartridges, it is necessary to build some. xe2x80x9cassembly clearancexe2x80x9d into the length of pressure vessels to assure multiple cartridges very close to the upper end of such tolerance can be accommodated. It has been determined that unusual wear in such seals can result from multiple cartridges shifting axially, i.e. shuttling, within a pressure vessel during the start-up and shutdown periods and that such problems need to be addressed to assure consistent high quality operation. These problems are further amplified in long length fiberglass-reinforced pressure vessels, e.g. over about 12 feet long, that, when subjected to superatmospheric pressure, have a tendency to grow slightly in length while a long membrane column has a tendency to shorten or collapse slightly when initially pressurized. Such changes in length under pressure would add to the assembly clearance and should also be taken into consideration lest they give rise to shuttling over a longer distance which would amplify the likelihood of resultant leakage of the higher pressure feed liquid into the permeate pathway.
Accordingly, in view of the foregoing, the search has continued for improvements which will assure continuous production of high quality permeate from systems employing a plurality of cylindrical cross-flow cartridges in a single pressure vessel.
Very broadly, the invention provides an arrangement whereby a biasing connection is made at one end of a pressure vessel designed to contain a plurality of cylindrical cartridges for use in a cross-flow filtration system. The biasing arrangement effectively compensates for assembly clearance, i.e. for tolerances in the axial length of spiral-wound cartridges which are evidenced at the time of initial loading of such cartridges in the pressure vessel, and for longitudinal growth of a polymeric pressure vessel as a result of initial pressurization to superatmospheric pressure as well as thereafter during extended operation under such pressure so as to maintain a stationary column. In addition, it is also able to absorb shocks that can be promulgated at the downstream end should there occur a sudden failure of a feed pump or some other happenstance that would cause immediate loss of liquid pressure at the feed inlet.
More specifically, the invention provides a pressure vessel for holding a plurality of cylindrical cartridges axially aligned in end-to-end columnar arrangement, such as spirally-wound semipermeable membrane cartridges designed for cross-flow separation which individually each create permeate streams that flow spirally inward to a central tube which has a porous sidewall in the region of the spiral wrapping. Both ends of the pressure vessel are closed, one of them by a removable end closure that provides full bore access, and the individual cartridges are respectively coupled to one another to create a combined-flow permeate passageway extending axially of the pressure vessel. The removable end closure is associated with an improved connection assembly which plugs the permeate passageway at the feed end of the pressure vessel and provides a biasing arrangement for applying axial force against the end cartridge, and therefore all of the cartridges in the column; this stabilizes the column of cartridges against axial movement during operation so that it remains stationary while such biasing arrangement is also operable to absorb any force that may be intermittently applied to said column of cartridges from the downstream end while resulting in only minimal axial movement of said column.
In a particular aspect, the invention provides a system for providing an aqueous permeate from an aqueous feed, such as brackish water or seawater, supplied thereto under superatmospheric pressure which system incorporates a generally tubular pressure vessel of circular interior cross-section, closed at the downstream end and having a plurality of spirally-wound cross-flow cylindrical cartridges of semipermeable membrane material located as a column in end-to-end relationship within the pressure vessel bore. A removable end closure closes the upstream end of the pressure vessel, and separate ports supply an aqueous feed under superatmospheric pressure, discharge a liquid concentrate stream from a location near the downstream end thereof and discharge the aqueous permeate. The improvement comprises the provision of a biasing arrangement which is associated with the removable end closure and which applies an axial force at the upstream end of the column of cartridges that stabilizes the column against any significant axial movement within the pressure vessel during times of pressurization and/or depressurization, and which is also effective to compensate for assembly clearance and for changes in the longitudinal length of the pressure vessel under operating pressure so as to assure a stationary column of cartridges.